Catalytic olefin polymerization catalyst system

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

A catalytically important system, methylalumoxane, was investigated by Zurek and Ziegler [88]. Computed 13C and proton chemical shifts of its Zr complexes were used to identify the active and dormant species of this black box activator of the dimethylcirconocene homogeneous olefin polymerization catalyst by comparison of the NMR parameters of the proposed species with experimental data (BP nonhybrid functional). 

Bis-Cp titanium derivatives supported on MgCl2/AlR (OEt)3 , activated with MAO or borate activators, have been used as catalytic systems for the polymerization of a-olefins.475 Supports of type MgCl2/AlR (OEt)3 have been shown to be effective for the immobilization and activation of Cp2TiCl2 and other single-site olefin polymerization catalysts without the use of MAO or a borate activator. Polyethylene with a spherical particle morphology and narrow molecular weight distribution has been obtained.1... 

The kinetics of olefin polymerization are the subject of several studles>104,153-156,162,182,221,226,240,241,246,252,255,266,28 12 and of an excellent book by Keii.17 The most relevant studies will be discussed below. However, we first note that the precise description of the kinetics of catalytic olefin polymerization under industrially relevant polymerization conditions has proved to be very difficult. For a given catalytic system, one has to identify all possible insertion, chain-release, and chain-isomerization reactions, and their dependence on the polymerization parameters (most importantly, temperature and monomer concentration). Once the kinetic laws for each elementary step have been determined, they have to be combined in one model in order to be able to predict the catalyst performance. This has been attempted for both ethylene and propylene polymerizations. The case of propylene polymerization with a chiral, isospecific zirconocene is shown in Figure 14.162... 

Rioka, M. Tsutsui, T. Ueda, T. Rashiwa, N. Stereospecific Polymerization of a-Olefin with an Ethylene Bis(l-indenyl)hafnium Dichloride and Methyl-aluminoxane Catalyst System. In Catalytic Olefin Polymerization, Studies in Surface Science and Catalysis-, Reii, T., Soga, R., Eds. Elsevier New York, 1990, p 483. 

In the following review we will focus on two classes of systems dispersed metal particles on oxide supports as used for a large variety of catalytic reactions and a model Ziegler-Natta catalyst for low pressure olefin polymerization. The discussion of the first system will focus on the characterization of the environment of deposited metal atoms. To this end, we will discuss the prospects of metal carbonyls, which may be formed during the reaction of metal deposits with a CO gas phase, as probes for mapping the environment of deposited metal atoms [15-19]. 

Eisch s work promoted investigation into the preparation of cationic metallocene complexes of Group 4 metals. Several preparative routes to cationic group 4 metallocene complexes are illustrated in Scheme II. Catalytic activities of some selected cationic metallocene complexes for the polymerization of a-olefins are summarized in Tables 5 and 6. The catalyst systems based on these cationic complexes are just as active as M AO-activated metallocene catalysts for the polymerization of a-olefins. 

A closer similarity exists between the C2-symmetric octahedral isospecific model sites, which have been proposed for the heterogeneous polymerization catalysts,13 15 and some slightly distorted octahedral metal complexes, including bidentate or tetradentate ligands, which have recently been described as active in isospecific olefin polymerization in the presence of MAO.128-130 In fact, all these catalytic systems can be described in terms of racemic mixtures of active species with A or A chiralities. 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

---